A rock jetty is a specialized, human-made structure built to manage the interaction between land and water, primarily in coastal environments. The structure extends from the shoreline into a body of water, such as an ocean, sea, or bay. Its function is to influence the movement of water and sediment, which is a significant factor in coastal stability and navigation. Jetties are commonly found near river mouths, harbor entrances, and inlets.
Defining the Structure and Materials
A rock jetty is a rigid, fixed barrier built perpendicular or near-perpendicular to the coastline. This orientation distinguishes it from similar coastal structures, such as a seawall (parallel to the shore) or a breakwater (parallel offshore to reduce wave energy). The structure is typically a solid construction, often called a rubble-mound structure, involving the strategic placement of massive stones.
The primary construction material is large, heavy, angular rock, frequently called riprap or armor stone, selected for its durability and resistance to harsh marine conditions. These stones are layered to create a stable mound that absorbs and deflects the constant force of waves. While rock is most common, some jetties incorporate reinforced concrete caissons, steel piles, or precast concrete blocks, depending on site conditions. The structure must be robust enough to withstand significant wave loads and tidal forces.
Primary Purpose in Coastal Engineering
The core functional goal for constructing jetties is to stabilize and maintain a navigable channel for vessels. Jetties are often built in pairs on either side of an inlet or harbor entrance to constrict the water flow and concentrate its energy. This concentrated flow increases the water velocity, which naturally scours the seabed and maintains a deeper channel depth.
By increasing the flow speed, the jetty system prevents shoaling—the deposition of sand and silt that can block a waterway. Without jetties, sediment carried by coastal currents would fill the navigation channel, requiring constant and costly dredging operations. The structure ensures safe passage for commercial and recreational traffic by preserving a stable, deep-water route.
Impact on Sediment and Shoreline Dynamics
The presence of a rock jetty fundamentally alters the natural flow of sediment along a coast by interrupting longshore drift. Longshore drift is the movement of sand parallel to the beach, caused by waves approaching the shore at an angle. The rigid, perpendicular structure of the jetty acts as a physical dam, blocking the transport of this sand.
As a result, sand accumulates on the side of the jetty facing the incoming current (the updrift side), causing the beach to widen through a process called accretion. However, because the jetty intercepts this sediment, the beach on the other side (the downdrift side) is starved of its natural sand supply. This sand starvation accelerates erosion on the downdrift shoreline, often leading to a loss of beach area and habitat. To mitigate this consequence, engineers employ sand bypassing, mechanically moving accumulated sand from the updrift side to the eroded downdrift side.